1. Technical Field
The present invention relates to a common mode filter and a manufacturing method thereof.
2. Background
Noise can be classified into two types according to the conduction mode. The first type is differential mode noise, which is conducted on a signal line and a ground line in opposite directions. The second type is common mode noise, which is conducted on all lines in the same direction.
Common mode filters can be used to suppress common mode noise on any line on which such noise is conducted. Traditionally, a common mode filter is comprised of components including an iron core and two coils wound around the iron core with the same winding number. When common mode current flows through the common mode filter, the two coils generate magnetic flux in the same direction such that the common mode filter exhibits high impedance and can suppress common mode noise.
To address the requirement of portable electronic apparatuses, thin film common mode filters have been developed. U.S. Pat. No. 7,145,427 B2 discloses one type of thin film common mode filter, which includes two coil conductor layers, two lead-out electrode layers, a plurality of insulation layers, and two magnetic layers. Each coil conductor layer includes a coil, and the two lead-out electrode layers are used to extend the inner ends of the two coils to an edge of the thin film common mode filter for external electrical connection. The insulation layers are used for electrically insulating the coil conductor layers and the lead-out electrode layers. The coil conductor layers, the lead-out electrode layers, and the insulation layers are disposed between two magnetic layers.
U.S. Pat. No. 6,356,181 B1 and U.S. Pat. No. 6,618,929 B2 respectively disclose a laminated common mode choke, which includes a pair of magnetic substrates and a plurality of insulting layers between the pair of magnetic substrates.
U.S. Pat. No. 7,453,343 B2 discloses another thin film type common mode filter, which comprises two coil conductor layers, two lead-out electrode layers, a plurality of insulation layers, a magnetic layer, and two magnetic substrates. Each coil conductor layer includes a coil, and each lead-out electrode layer is configured to extend the inner end of the corresponding coil. The insulation layer is configured to electrically insulate the coil conductor layer and the lead-out electrode layer. The coil conductor layers, the lead-out electrode layers, and the insulation layers are disposed between the two magnetic substrates. The magnetic layer is disposed between the two magnetic substrates and attached to one magnetic substrate by a glue layer. The patent discloses that when the coil conductor width is reduced to 36 micrometers or less, the cutoff frequency of a transmission signal can be effectively increased to at least 2.4 GHz (800 MHz transmission×3). However, because the coil conductor is narrow, the coil has high resistance such that the coil conductor cannot be formed thin. As a result, the thin film type common mode filter has a significant thickness.
The thin film type common mode filter utilizes the magnetic substrates and the magnetic layer to concentrate magnetic fields and reduce the dimension of the thin film type common mode filter. However, the magnetic substrates and the magnetic layer can easily cause energy loss, consequently causing the common mode filter to have a low quality factor. In particular, when the common mode filter is used in a radio frequency circuit, the energy loss becomes more significant. In addition, the common mode filter using magnetic substrates and a magnetic layer may have insertion loss of greater than −20 dB.
In order to reduce the shortcomings of the common mode filter using magnetic substrates and a magnetic layer, U.S. Pat. No. 7,821,368 B1 discloses a thin film type common mode noise filter and a fabrication method. The fabrication method forms a structure including several electric insulation layers, coil lead layers and main coil layers on an insulation substrate by lithography processes, physical vapor deposition, etching processes or other chemical processes, and subsequently covers the structure with an electric insulation gluing layer and a magnetic material layer. Such design of thin film type common mode noise filter can be formed at low production cost and has improved filtering characteristics. Although the thin film type common mode noise filter can have better filtering characteristics, the thin film type common mode noise filter has a filter bandwidth narrower than that of the common mode filtering adopting magnetic substrates.
Therefore, conventional thin film common mode filters cannot simultaneously have improved noise suppression effectiveness and a wide rejection bandwidth.